Blood pressure measuring devices have a pressure system with a cuff which can be inflated, a pressure generating means, a pressure measuring device and an exhaust valve for decreasing the overpressure in the pressure system.
In one known exhaust valve, an outlet line which opens into free space is provided on a valve housing. In the course of the outlet line a valve seat is provided which is formed by a hollow frustoconical surface which annularly surrounds the outlet line. A conical valve body, which is guided so as to be movable back and forth relative to the valve seat along the longitudinal axis of the valve seat, cooperates with the valve seat. For this purpose, the valve body is connected to a screw cap which is adjustably guided by means of a thread concentrically to the longitudinal axis of the valve seat and consequently of the valve body. In order to close the pressure system as it is pumped up, the valve body is pressed by means of the screw cap against the valve seat through a screwing movement. Since a substantial transfer of force takes place through the threaded connection between the screw cap and the valve housing, the vertex angle of the conical surfaces of the valve seat and the valve body must not be too small in order to avoid binding or wedging of the valve body due to the friction between the conical surfaces. For this reason the vertex angle generally amounts to sixty degrees. Nevertheless, a clearly discernible binding friction or sticking arises between the two conical surfaces if the screw cap is closed with a certain degree of force to achieve a secure closing of the exhaust valve.
Due to the relatively oblique vertex angles of the two sealing surfaces, a relatively large change in cross section of the cross-sectional area which forms the outlet of the outlet valve occurs, even when there is only a very slight axial movement of the valve body with respect to the valve seat. Because at the beginning of the blood pressure measuring operation, the internal pressure in the pressure system has its highest value, a relatively high exhaust rate occurs when the outlet valve is opened which leads to a correspondingly rapid loss of pressure due to the limited volume of the pressure system. This rapid loss of pressure makes it more difficult to reliably determine the systolic blood pressure value, which must be read from the pressure measuring device as soon as the first sound of the Korotkoff flow arises in the artery underneath the point which was blocked off by the cuff at the higher overpressure which previously prevailed. The fine feel at the very beginning of the measurement operation is additionally made more difficult by the fact that the screw cap must be turned with a large initial twisting force due to the previously described sticking or frictional binding between the two sealing surfaces until the higher frictional holding force between the sealing surfaces is overcome and a lower sliding friction value is achieved. Since this transition from binding friction to sliding friction occurs abruptly, the higher initial twisting force applied by the fingers leads to the screw cap being turned beyond the point which would be necessary to have a fine "feel" of the release of pressure. Thus after this initial turn, the screw cap with the valve body must therefore first be turned back somewhat in the closure direction again in order to obtain the desired rate of escape for the air exiting from the pressure system.
In another known blood pressure measuring apparatus described in published German Patent Application No. DE-OS 21 28 295, the fine feel of the release of pressure even in the initial stage of the blood pressure measurement operation is made possible by using a relatively slender valve cone. This valve cone is actuated by means of a pivot key in order to achieve the larger adjusting movement which is required by the slender cone angle. Even though the actuation of the exhaust valve by means of the pivot key is significantly more convenient and a very fine feel of the reduction in pressure is possible because of the more favorable pressure release behavior of this valve, there are nonetheless persons who often use a blood pressure measuring device for professional reasons or other reasons and who do not want to switch from the familiar blood pressure measuring apparatus with the screw cap which they have used in the past to the other type of exhaust valve with the pivot key.